BEAVER DAMS. 105 



Esconauba after it has passed dam No. 13, and on 

 Carp River after passing dam No. 39. The channel 

 of the first-named stream will then average seventy 

 feet in width, with vertical banks from three to four 

 feet high, and with a depth of water of about twenty 

 inches at its lowest stages, and in its shallowest parts. 

 Through the level areas it moves also with a sluggish 

 current. It will be seen, therefore, that in building 

 a dam across such a channel, it must be done in deep 

 water as compared with brooks; and further than this, 

 that the difficulty of construction increases with the 

 increase of the depth of the water, until it finally 

 becomes insurmountable. For this reason there are 

 no dams on the Carp below No. 50, and none on the 

 Esconauba below the junction of the Ely Branch 

 with the main stream. There is no instance within 

 the area represented by the map where a dam has 

 been constructed across a stream having a greater 

 depth than two feet at the site of the structure when 

 the water is at its lowest level. It thus becomes 

 apparent that beaver dams are necessarily confined 

 to the sources of the principal rivers and to the small 

 tributaries which flow into them along their courses; 

 and that some change in the character of the dams 

 would be rendered necessary by the transformations 

 which occur with their increase in size or depth. 

 Where beavers inhabit rivers too large for dams, 

 they burrow in their banks, for which reason they 

 are distinguished by the trappers under the name of 

 bank beavers. These general considerations will serve 

 to explain the manner in which given districts are 

 occupied by beavers; the circumstances which render 

 some localities more favorable than others ; and the 



